US11644520B2 - Systems and methods for magnetic resonance based skull thermometry - Google Patents
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4808—Multimodal MR, e.g. MR combined with positron emission tomography [PET], MR combined with ultrasound or MR combined with computed tomography [CT]
- G01R33/4814—MR combined with ultrasound
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
- A61N7/00—Ultrasound therapy
- A61N7/02—Localised ultrasound hyperthermia
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/3804—Additional hardware for cooling or heating of the magnet assembly, for housing a cooled or heated part of the magnet assembly or for temperature control of the magnet assembly
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4804—Spatially selective measurement of temperature or pH
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4816—NMR imaging of samples with ultrashort relaxation times such as solid samples, e.g. MRI using ultrashort TE [UTE], single point imaging, constant time imaging
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B18/00—Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
- A61B2018/00636—Sensing and controlling the application of energy
- A61B2018/00773—Sensed parameters
- A61B2018/00791—Temperature
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
- A61B90/36—Image-producing devices or illumination devices not otherwise provided for
- A61B90/37—Surgical systems with images on a monitor during operation
- A61B2090/374—NMR or MRI
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
- G01R33/4824—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a non-Cartesian trajectory
- G01R33/4826—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a non-Cartesian trajectory in three dimensions
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/50—NMR imaging systems based on the determination of relaxation times, e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences
Definitions
- Magnetic resonance guided focused ultrasound can enable brain surgery with focused ultrasound (FUS) waves mechanically perturbing or heating brain tissue.
- the procedure can be performed by placing a patient's head into a FUS helmet composed of a number of transducers (e.g. 1024 transducers).
- transducers e.g. 1024 transducers.
- a surgeon can destroy the targeted tissue to millimeter precision at ablative temperatures (55-60° C.) with no damage to surrounding tissue and thereby treat different disorders.
- An MRI is used to image the target and to determine the coordinates for the FUS system as well as to monitor the effect of the treatment through changes in T1, T2, and diffusion of the target.
- MRgFUS has been successfully applied to treat patients with essential tremor (ET).
- ET essential tremor
- Patients with ET have a tremor typically affecting their hands and quality of life making functional activities such as drinking a glass of water, dressing, or writing very difficult.
- Ablation of the thalamus in the brain helps to suppress the tremor observed during and immediately after the procedure.
- MRgFUS in the brain can also treat the symptoms of Parkinson's disease, neuropathic pain, and brain tumors.
- MRgFUS is a rapidly growing technology in interventional radiology and functional neurosurgery, there remain many technical challenges to be solved so that MRgFUS can be a widespread treatment option for neuropathology.
- Examples of medical applications include FDA approved treatment for Parkinson's disease and essential tremor and many other disorders in the research stage such as neuropathic pain, depression, and obsessive-compulsive disorder.
- One challenge to treatment efficacy is posed by the skull. Its high absorption of ultrasound waves creates difficulties, one of which is skull heating. Damage from skull heating has been observed in several patients. Though damage has not been shown to be harmful, it may be linked to problems such as headaches during treatment. Temperature monitoring of the skull would increase treatment safety, enable further development of MRgFUS therapy to non-central brain targets, and potentially speed up treatment by decreasing waiting time between sonications for patients.
- MRI based thermometry is well suited for this task as monitoring of the brain temperature is already done by MRI.
- Bone can attenuate ultrasound energy 20 times more efficiently than soft tissue. Heating of the skull during FUS therapy can be a major concern and limit the amount of acoustic energy that can be safely transmitted into the brain and constrain which parts of the brain can be targeted. Targets away from the center of the brain lead to more skull heating. Despite current clinical precautions such as cooling the scalp actively with circulating water, there is still potential for injury. A recent study has shown that MRgFUS led to unintended skull lesions in 16 out of 40 MRgFUS procedures. Real-time skull thermometry can validate proposed skull heating models and prevent unintended injury to patients. It can also make treatment faster as surgeons can wait 6-15 minutes for the skull to cool in between sonications during the long (e.g.
- the present disclosure relates to systems, methods, and computer-readable medium for magnetic resonance (MR) based thermometry.
- the present disclosure relates to a method which, in one embodiment includes acquiring, by a variable flip-angle (VFA) T1 mapping sequence, MR data in an area of interest of a subject corresponding to cortical bone of at least part of the skull that is heated by the application of focused ultrasound (FUS) to a selective portion of the brain of the subject, where the MR data includes a plurality of T1 values over time that include a first point in time and a second, later point time, and where the acquisition of the MR data includes applying an accelerated three-dimensional (3D) ultra-short (UTE) spiral acquisition sequence with a nonselective excitation pulse.
- VFA variable flip-angle
- the method also includes determining, based at least in part on a mathematical relationship established by T1 mapping thermometry produced according to the T1 mapping sequence, a temperature change in the cortical bone that occurs between the first point in time and the second point in time, and where the temperature change is caused at least in part by a change in the applied FUS.
- the present disclosure relates to a system for magnetic resonance (MR) based thermometry, which in one embodiment includes a magnetic resonance imaging (MRI) device configured to acquire, by implementing a variable flip-angle (VFA) T1 mapping sequence, MR data in an area of interest of a subject corresponding to cortical bone of at least part of the skull that is heated by the application of focused ultrasound (FUS) to a selective portion of the brain of the subject, where the MR data includes a plurality of T1 values over time that include a first point in time and a second, later point time, and where the MRI device is further configured to acquire the MR data using an accelerated three-dimensional (3D) ultra-short (UTE) spiral acquisition sequence with a nonselective excitation pulse.
- MRI magnetic resonance imaging
- VFA variable flip-angle
- the system also includes a processor coupled to the MRI device and configured to cause the system to perform functions that include determining, based at least in part on a mathematical relationship established by T1 mapping thermometry produced according to the T1 mapping sequence, a temperature change in the cortical bone that occurs between the first point in time and the second point in time, and where the temperature change is caused at least in part by a change in the applied FUS.
- the present disclosure relates to a non-transitory computer-readable medium having stored instructions that, when executed by one or more processors of a computing device, cause a system for magnetic resonance (MR) based thermometry to perform specific functions.
- the specific functions performed include: acquiring, by a variable flip-angle (VFA) T1 mapping sequence, MR data in an area of interest of a subject corresponding to cortical bone of at least part of the skull that is heated by the application of focused ultrasound (FUS) to a selective portion of the brain of the subject, where the MR data includes a plurality of T1 values over time that include a first point in time and a second, later point time, and where the acquisition of the MR data includes applying an accelerated three-dimensional (3D) ultra-short (UTE) spiral acquisition sequence with a nonselective excitation pulse; and determining, based at least in part on a mathematical relationship established by T1 mapping thermometry produced according to the T1 mapping sequence, a temperature change in the cortical bone
- FIG. 1 is a flow diagram showing operations of a method for performing accelerated T1 thermometry in accordance with an embodiment of the present disclosure.
- FIG. 2 is a table illustrating non-limiting examples of clinical parameters that can be achieved using embodiments of the present disclosure.
- FIG. 3 is an illustration of the UTE VIBE sequence for magnetic resonance data acquisition.
- FIG. 6 illustrates a non-limiting example of a UTE VIBE K-space Trajectory. Uniform spiral density.
- FIG. 7 illustrates the effect of T2 decay during readout.
- FIG. 8 illustrates an example of whole-head in-vivo UTE data acquired in 67 seconds.
- TR 10 ms.
- TE 50-370 us.
- FIG. 9 illustrates ex-vivo skull high-resolution UTE data.
- TR 11 ms.
- TE min 50 us.
- the second illustration corresponds to data obtained at TE of 2.5 ms for late-TE comparison.
- FIGS. 10 A- 10 C illustrate the accuracy of T1 thermometry, wherein FIG. 10 A illustrates the accuracy of the T1-Mapping Method using IR, FIG. 10 B illustrates the bone thermometry method using VFA, and FIG. 10 C illustrates a comparison of the accuracy of T1 derived from VFA to T1 from IR and compares the methods to the expected result.
- FIGS. 11 A- 11 B illustrate an experimental setup and result for a water bath cooling experiment, wherein FIG. 11 A illustrates the location of two thermocouples in a sample, and FIG. 11 B illustrates the experimental result.
- FIGS. 12 A- 12 B illustrate an experimental setup and result for a water bath heating experiment, wherein FIG. 12 A illustrates the experimental configuration; and FIG. 12 B illustrates the experimental result.
- FIGS. 13 A- 13 B illustrate the relationships between different T1 signals and temperature in bone for various experiments, wherein FIG. 13 A illustrates the relationship between changes in T1-weighted signal vs. temperature in bone, and FIG. 13 B illustrates the relationship between a change in T1 vs. temperature in Bone.
- FIGS. 14 A- 14 B illustrate experimental results.
- FIG. 14 A illustrates the measured T1-weighting sensitivity for different flip angles.
- FIG. 14 B illustrates T1-weighted Signal vs. Temperature in Cow Bone, where the signal behavior with temperature is nonlinear, even at higher flip angles (43.5°).
- FIGS. 15 A- 15 B illustrate an experimental result corresponding to a location on a particular bone, wherein FIG. 15 A illustrates the location the data was sampled from, and FIG. 15 B illustrates the relationship between the T1 signal and temperature.
- FIGS. 17 A- 17 B illustrate an sVFA acceleration experiment wherein FIG. 17 A illustrates an experimental configuration; FIG. 17 B illustrates the relationship between bone T1 and temperature, and FIG. 17 C illustrates the relationship between NiCl 2 and temperature.
- FIG. 17 A illustrates an experimental configuration
- FIG. 17 B illustrates the relationship between bone T1 and temperature
- FIG. 17 C illustrates the relationship between NiCl 2 and temperature.
- FIG. 17 B illustrates how the sVFA results show a smaller slope and can underestimate the T1
- FIG. 17 C illustrates how for NiCl 2 nominal VFA shows good linearity and slope.
- sVFA without correction can overestimate T1 especially at higher Temperatures.
- sVFA with correction decreases overestimation but not completely.
- FIGS. 18 A- 18 B illustrate T1 vs. temperature for an ex-vivo human skull, wherein FIG. 18 A illustrates how under sampling can decrease the signal amplitude and slightly underestimate T1; and FIG. 18 B illustrates how the results can be ROI-dependent, but T1 vs. temperature can show a consistently positive slope of varying magnitude.
- FIG. 19 illustrates the temperature vs. time graph for an embodiment of the present disclosure tested using a sample of bone in fomblin.
- FIGS. 20 A- 20 C illustrate an experimental setup and result where a linear T1 change was detected with temperature in the target.
- FIG. 20 A illustrates the experimental configuration
- FIG. 20 B illustrates the linear change in T1 and temperature
- FIG. 20 C illustrates the T1 difference map.
- the T1 difference map shows heating occurred at the bottom of the bone.
- the data was temporally averaged with a time window of 2 as a less optimal coil was used in this experiment than in other water bath experiments described herein.
- FIG. 21 illustrates fully sampled images (top row) and under-sampled images (bottom row) in which the under-sampled images can be generated approximately twice as fast, even though the image quality is comparable between the images.
- FIGS. 22 A- 22 D illustrate results before and after the RF Flip Angle Calibration, wherein FIG. 22 A illustrates results before RF Flip Angle Calibration without the FUS transducer setup;
- FIG. 22 B illustrates results before RF Flip Angle Calibration without the FUS transducer setup
- FIG. 22 C represents a plot of signal vs. voltage after RF Flip Angle Calibration
- FIG. 22 D illustrates the results with the FUS transducer setup after RF Flip Angle Calibration.
- FIG. 23 is a system diagram illustrating an imaging system capable of implementing aspects of the present disclosure in accordance with one or more embodiments.
- FIG. 24 is a diagram showing an example embodiment of a system with thermal therapy used with MRI, which is capable of implementing aspects of the present disclosure in accordance with one or more embodiments.
- FIG. 25 is a computer architecture diagram showing a computing system capable of implementing aspects of the present disclosure in accordance with one or more embodiments.
- the disclosed technology relates to systems, methods, and computer-readable medium for magnetic resonance based skull thermometry.
- example embodiments of the disclosed technology are explained in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the disclosed technology be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The disclosed technology is capable of other embodiments and of being practiced or carried out in various ways.
- a “subject” may be any applicable human, animal, or other organism, living or dead, or other biological or molecular structure or chemical environment, and may relate to particular components of the subject, for instance specific organs, tissues, or fluids of a subject, may be in a particular location of the subject, referred to herein as an “area of interest” or a “region of interest.”
- Embodiments of the present disclosure include MRI-based thermometry techniques.
- the MRI-based thermometry technique is adapted to measure heating in the skull of a human patient during a focused ultrasound (FUS) treatment.
- FUS focused ultrasound
- FIG. 23 is a system diagram illustrating an imaging system capable of implementing aspects of the present disclosure in accordance with one or more example embodiments.
- FIG. 23 illustrates an example of a magnetic resonance imaging (MRI) system 100 , including a data acquisition and display computer 150 coupled to an operator console 110 , an MRI real-time control sequencer 152 , and an MRI subsystem 154 .
- the MRI subsystem 154 may include XYZ magnetic gradient coils and associated amplifiers 168 , a static Z-axis magnet 169 , a digital RF transmitter 162 , a digital RF receiver 160 , a transmit/receive switch 164 , and RF coil(s) 166 .
- the MRI subsystem 154 may be controlled in real time by control sequencer 152 to generate magnetic and radio frequency fields that stimulate magnetic resonance phenomena in a subject P to be imaged, for example to implement magnetic resonance imaging sequences in accordance with various embodiments of the present disclosure.
- Reconstructed images such as contrast-enhanced image(s) of an area of interest A of the subject P may be shown on display 170 .
- the area of interest A shown in the example embodiment of FIG. 23 corresponds to a head region of subject P, but it should be appreciated that the area of interest for purposes of implementing various aspects of the disclosure presented herein is not limited to the head area. It should be recognized and appreciated that the area of interest in various embodiments may encompass various areas of subject P associated with various physiological characteristics, such as, but not limited to the head and brain region, chest region, heart region, abdomen, upper or lower extremities, or other organs or tissues. Various aspects of the present disclosure are described herein as being implemented on portions of the skeletal system of human subjects, for example cortical bone tissue.
- One or more data acquisition or data collection steps as described herein in accordance with one or more embodiments may include acquiring, collecting, receiving, or otherwise obtaining data such as imaging data corresponding to an area of interest.
- data acquisition or collection may include acquiring data via a data acquisition device, receiving data from an on-site or off-site data acquisition device or from another data collection, storage, or processing device.
- data acquisition or data collection devices of a system in accordance with one or more embodiments of the present disclosure may include any device configured to acquire, collect, or otherwise obtain data, or to receive data from a data acquisition device within the system, an independent data acquisition device located on-site or off-site, or another data collection, storage, or processing device.
- FIG. 24 is a diagram showing an embodiment of a system with focused ultrasound (FUS) used with MRI, each of which is capable of implementing aspects of the present disclosure in accordance with one or more embodiments.
- the MRI system may comprise one or more components of the system 100 shown in FIG. 23 .
- RF coils 222 , gradient coils 224 , static Z axis magnet 226 , and magnetic housing 216 surround the patient P when the patient is positioned on the table 214 inside of the MRI bore 218 .
- a controller 212 communicates with MRI system electronics 210 as well as the FUS device ( 225 ).
- the MRI system electronics 210 can include one or more components of the MRI subsystem 154 shown in FIG. 23 .
- a user computer (not shown) may communicate with the controller 212 for control of the MRI system and FUS device functions.
- a type of FUS device 225 surrounds the patient's head, as may be used for thermal therapy applied to tissues of or near the brain.
- the device 225 may have multiple ultrasound transducers for applying focused energy to particular target areas of interest of the head of the patient.
- the device 225 can be configured to apply localized energy to heat a targeted region within the area of interest A which includes tissues of or near the brain. As a result, heating may occur in bone tissues, such as that of the skull.
- the MRI components of the system (including MRI electronics 210 ) are configured to work within a larger MRI system to acquire magnetic resonance data and for reconstructing images of all or regions of the area of interest as well as temperature-related data.
- the temperature data may include a temperature at a targeted region and/or a temperature at a reference region. The temperature data may be used to monitor the effectiveness and safety of the thermal therapy treatment and adjust treatment settings accordingly.
- the targeted region may include bone tissue, which as described above, has a short T2/T2*.
- Control of the application of the focused energy via the controller 212 may be managed by an operator using an operator console (e.g., user computer).
- the controller 212 (which, as shown is also coupled to MRI electronics 210 ) may also be configured to manage functions for the application and/or receiving of MR signals.
- the controller 212 may be coupled to a control sequencer such as the control sequencer 152 shown in FIG. 23 .
- FUS device 225 shown in the embodiment of FIG. 24 utilize ultrasound transducer(s) as the source for delivering localized energy to an area of interest, it should be appreciated that other types of devices may alternatively be used without departing from the patentable scope of the present disclosure.
- Other possible types of thermal treatment/application devices that may be utilized include laser and/or RF ablation devices, or other devices adapted to heat a target tissue.
- FIG. 25 is a computer architecture diagram showing a computing system capable of implementing aspects of the present disclosure in accordance with one or more embodiments described herein.
- a computer 300 may be configured to perform one or more specific steps of a method and/or specific functions for a system.
- the computer may be configured to perform one or more functions associated with embodiments illustrated in one or more of FIGS. 1 - 24 .
- the computer 300 may be configured to perform aspects described herein for implementing the pulse sequences shown and for various aspects of magnetic resonance imaging and related signal and temperature monitoring shown in FIGS. 1 - 24 .
- the computer 300 may be implemented within a single computing device or a computing system formed with multiple connected computing devices.
- the computer 300 may be configured to perform various distributed computing tasks, in which processing and/or storage resources may be distributed among the multiple devices.
- the data acquisition and display computer 150 and/or operator console 110 of the system shown in FIG. 23 , and the controller 212 and/or MRI electronics 210 of the system shown in FIG. 24 may include one or more components of the computer 300 .
- the computer 300 includes a processing unit 302 (“CPU”), a system memory 304 , and a system bus 306 that couples the memory 304 to the CPU 302 .
- the computer 300 further includes a mass storage device 312 for storing program modules 314 .
- the program modules 314 may be operable to perform functions associated with one or more embodiments described herein. For example, when executed, the program modules can cause one or more medical imaging devices, localized energy producing devices, and/or computers to perform functions described herein for implementing the pulse sequence shown in FIG. 3 , the method shown in FIG. 1 , and for various aspects of magnetic resonance imaging and related signal and temperature monitoring and analysis shown in FIGS. 1 - 24 .
- the program modules 314 may include an imaging application 318 for performing data acquisition and/or processing functions as described herein, for example to acquire and/or process image data corresponding to magnetic resonance imaging of an area of interest.
- the computer 300 can include a data store 320 for storing data that may include imaging-related data 322 such as acquired data from the implementation of magnetic resonance imaging pulse sequences in accordance with various embodiments of the present disclosure.
- the mass storage device 312 is connected to the CPU 302 through a mass storage controller (not shown) connected to the bus 306 .
- the mass storage device 312 and its associated computer-storage media provide non-volatile storage for the computer 300 .
- computer-storage media can be any available computer storage media that can be accessed by the computer 300 .
- computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-storage instructions, data structures, program modules, or other data.
- computer storage media includes, but is not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, digital versatile disks (“DVD”), HD-DVD, BLU-RAY, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer 300 .
- Computer storage media “computer-readable storage medium” or “computer-readable storage media” as described herein do not include transitory signals.
- the computer 300 may operate in a networked environment using connections to other local or remote computers through a network 316 via a network interface unit 310 connected to the bus 306 .
- the network interface unit 310 may facilitate connection of the computing device inputs and outputs to one or more suitable networks and/or connections such as a local area network (LAN), a wide area network (WAN), the Internet, a cellular network, a radio frequency (RF) network, a Bluetooth-enabled network, a Wi-Fi enabled network, a satellite-based network, or other wired and/or wireless networks for communication with external devices and/or systems.
- LAN local area network
- WAN wide area network
- RF radio frequency
- Bluetooth-enabled network a Wi-Fi enabled network
- satellite-based network or other wired and/or wireless networks for communication with external devices and/or systems.
- the computer 300 may also include an input/output controller 308 for receiving and processing input from any of a number of input devices.
- Input devices may include one or more of keyboards, mice, stylus, touchscreens, microphones, audio capturing devices, and image/video capturing devices.
- An end user may utilize the input devices to interact with a user interface, for example a graphical user interface, for managing various functions performed by the computer 300 .
- the input/output controller 308 may be configured to manage output to one or more display devices for displaying visually representations of data, such as display monitors/screens that are integral with other components of the computer 300 or are remote displays.
- the bus 306 may enable the processing unit 302 to read code and/or data to/from the mass storage device 312 or other computer-storage media.
- the computer-storage media may represent apparatus in the form of storage elements that are implemented using any suitable technology, including but not limited to semiconductors, magnetic materials, optics, or the like.
- the computer-storage media may represent memory components, whether characterized as RAM, ROM, flash, or other types of technology.
- the computer storage media may also represent secondary storage, whether implemented as hard drives or otherwise. Hard drive implementations may be characterized as solid state, or may include rotating media storing magnetically-encoded information.
- the program modules 314 which include the imaging application 318 , may include instructions that, when loaded into the processing unit 302 and executed, cause the computer 300 to provide functions associated with one or more embodiments illustrated in FIGS. 1 - 24 .
- the program modules 314 may also provide various tools or techniques by which the computer 300 may participate within the overall systems or operating environments using the components, flows, and data structures discussed throughout this description.
- T1 recovery results from dipolar magnetic field interactions between the two hydrogen protons in the same water molecule and also from inter-molecular interactions.
- the field fluctuations are characterized by the frequency spectral density, J(w), which depends on motion as well. For example, free water exhibits fast motion and has a narrow J(w), so its T1 values are long.
- T2 is also dependent on correlation time:
- T 1 2 ⁇ ⁇ 2 ⁇ B l ⁇ o ⁇ c 2 3 ⁇ ⁇ c ( T ) 1 + ⁇ o 2 ⁇ ⁇ c ( T ) 2
- T 1 ⁇ ⁇ c ⁇ ⁇ c ⁇ c is also inversely proportional to temperature, so T1 also approximately increases linearly with temperature within the clinical regime:
- T T 1 ( T ) - T 1 ( T ref ) m 1 + T ref
- T1 thermometry A difficulty of T1 thermometry is caused by the tissue dependence of m 1 . Unlike the ⁇ constant from PRF which was tissue-independent, m 1 has high sample variability. T1 changes for not-fatty tissue are not always reversible, especially if tissue coagulation occurs. However, T2 thermometry also has a variable tissue dependent factor m 2 .
- T1 is less sensitive to the B 0 -field of the scanner compared to T2* and does not require a refocusing pulse compared to T2, it is very sensitive to a non-ideal slice profile which occurs when the small flip angle approximation does not apply. If the slice profile is non-ideal, then the T1 measurements can be erroneous. There are some methods for correcting for non-ideal slice profile, but they are still not fully reliable.
- T1-weighted thermometry While work in T1-weighted thermometry may show promise for some applications, the repeatability of T1-weighted thermometry has not been investigated. T1-mapping has more potential to be repeatable and easier to calibrate, but suffers from requiring more acquisition time compared to T1-weighted thermometry. T1-weighted signal acquired with a volumetric spiral sequence decreases linearly with increasing temperature and can meet the clinical constraints in a repeatable way.
- a challenge of T1-mapping is that at least two flip angles of data must be acquired per temperature point doubling acquisition time.
- acceleration techniques can be employed to make T1 thermometry viable.
- magnetic resonance (MR) data is by a variable flip-angle (VFA) T1 mapping sequence, MR data in an area of interest of a subject.
- the area of interest of the subject can be any part of the subject's body on which FUS is applied.
- FUS focused ultrasound
- the skull of the subject i.e. a human patient receiving the treatment
- the area of interest can include corresponding cortical bone of at least part of the skull of the subject that is heated by the application of FUS to treat a selective portion of the brain of the subject.
- the skull has several important properties relevant for choosing MR sequence parameters. There can be very little water in the skull (which can impact proton density) which can decrease the amount of MR signal available. This can be mitigated using high SNR techniques. Water in the skull exists as free water and bound water. Bound water has a very short transverse relaxation time (T2) on the order of ⁇ 100 us. The echo time therefore many need to be on the order of ⁇ 100 us as well. Conventional MRI can be too slow to measure the transverse magnetization of bone before it decays away. Therefore a UTE (ultra-short echo time) sequence can be employed (e.g. a UTE sequence originally designed to measure lung tissue). On average, the skull is 5.58-8.17 mm thick, which can require good imaging resolution (e.g.
- a resolution of 5 ⁇ 5 ⁇ 5 mm a resolution of 5 ⁇ 5 ⁇ 5 mm). Further, its thickness varies from location to location and between patients.
- a large field of view can be used in some embodiments of the present disclosure.
- a non-selective 3D sequence can be used in embodiments of the present disclosure to achieve a large field of view.
- the skull's bone can be similar to a ceramic material functioning as a thermal insulator preventing heat flow from the scalp into the brain and vice versa, and it has a cooling time constant estimated to be on the order of minutes. Therefore, the temporal resolution should be short compared to the cooling time of the skull, for example some embodiments of the present disclosure can achieve a temporal resolution of 90 s or less.
- a table of values showing non-limiting examples of design/clinical parameters is shown in FIG. 2 , including the above skull parameters and other design constraints of MRI-based thermometry.
- the MR data in step 102 can include a plurality of T1 values over time that include a first point in time and a second, later point time, where the acquisition of the MR data comprises applying an accelerated three-dimensional (3D) ultra-short (UTE) spiral acquisition sequence with a nonselective excitation pulse, and where the acceleration of the accelerated 3D UTE spiral acquisition sequence comprises the use of at least one of partial kz acquisition and variable density of spiral interleaves.
- 3D three-dimensional
- UTE ultra-short
- the acquisition of MR data can be performed using the UTE VIBE sequence, which is illustrated in FIG. 3 .
- the UTE VIBE sequence is a spoiled GRE sequence suitable for T1-based contrast imaging and is ultimately very fast.
- An alternative sequence that can be used in some embodiments of the present disclosure is the AWSOS (acquisition-weighted stack of spirals) sequence which uses a stack of spirals to accelerate in-plane data collection, variable-duration slice encoding, and a movable spiral readout achieving an echo time of 608 us.
- Differences between UTE VIBE and AWSOS include that the UTE VIBE is non-selective with a rectangular RF pulse, and the min TE is less than 100 us.
- UTE VIBE sequence was developed for breath-hold UTE lung imaging.
- UTE VIBE has the following advantages for bone thermometry: (1) an ultra-short echo time limited only by the duration of a rectangular pulse; (2) a spiral readout enabling a highly efficient short readout duration which starts at the center of k-space; (3) non-selective (3D) excitation. While the present disclosure refers to UTE VIBE as an exemplary sequence, it should be understood that the use of other sequences is contemplated by the present disclosure.
- M xy is the measured signal
- M o is the thermal equilibrium magnetization
- ⁇ is the flip angle
- T R is the repetition time.
- the e ⁇ T R /T 1 (T) term provides the T1-weighting on the signal. If the TE is sufficiently short, then the e ⁇ TE/T 2 *(T) term is negligible.
- T1 can then be estimated by using linear least squares fitting on Eq. 1 from signal from two flip angles.
- the two optimal flip angles are calculated by using propagation of errors to minimize an expression of uncertainty in quantitative VFA T1 mapping occurring when the signal is 0.71 of the Ernst angle signal (maximum signal).
- RF spoiler which prevents coherences from previous TR (stored in Mz) from contributing to the current TR's signal.
- the G z spatial encoding is one of the strengths of this sequence in minimizing echo time.
- Z-information is phase-encoded with a G z gradient after the RF pulse and before the readout spiral.
- Each TR corresponds to a selected k-z plane in k-space, so that the third dimension is sampled traditionally in the Cartesian way, whereas k-space in k x , k y dimensions is sampled using spirals.
- the sampling trajectory is a stack of spirals.
- the variable duration of the G z gradient leads to a variable echo time.
- the echo time depends on the length of the G z -phase encode gradient and is thus variable as described above and shown in FIG. 4 A .
- the maximum echo time is 373 us for the highest kz plane of data. Because most of the signal energy comes from the center of k-space, the effective echo time is close to the minimum echo time.
- variable echo time leads to blurring as the longer echo time corresponds to more T2-decay (attenuation) of the higher spatial frequencies ( FIG. 4 B ), in which the signal depends on the echo time, as described originally by Qian et al:
- FIGS. 5 A- 5 B For species with a T2 of 450 us, a blur of 0.6 mm is predicted to occur for UTE VIBE which meets the goal for human imaging and this blur is illustrated in FIGS. 5 A- 5 B .
- spirals can be technically difficult to implement on a scanner, can require special reconstruction techniques, and can be sensitive to off-resonance, they have many advantages, such as (1) reducing acquisition time due to efficient k-space coverage; (2) having a large SNR by starting acquisition at the center of k-space, which is also an advantage for ultra-short echo time sequences; (3) being robust against motion in dynamic MRI; (4) allowing real-time MRI with high in-plane resolution; and (5) being less sensitive to aliasing. For these reasons, spirals are a viable option for bone thermometry, which requires ultra-short echo time, high SNR, and rapid image acquisition.
- the k-space spiral trajectory as implemented in a non-limiting example of an MRI scanner that can be used in an embodiment of the present disclosure is shown in FIG. 6 .
- the UTE VIBE has the advantage of imaging a large volume (240 mm 3 ) under 90 s ( FIG. 8 ) making it rapid enough for skull thermometry during MRgFUs.
- FIG. 9 illustrates MR data acquired from an ex vivo skull at high resolution.
- T1-mapping thermometry can take twice as long as T1-weighted thermometry, the T1 vs. temperature trend is much more reliable and linear.
- the advantages of spiral MRI it is possible to accelerate T1-mapping to meet the clinical constraints (e.g. the non-limiting constraints illustrated in FIG. 2 ).
- the MR data acquisition sequence used in step 102 can be accelerated to conform to clinical constraints (e.g. the constraints shown in FIG. 2 ).
- clinical constraints e.g. the constraints shown in FIG. 2 .
- FIG. 2 shows a resolution of as a target, ( ⁇ 5 ⁇ 5 ⁇ 5 mm), however, embodiments of the present disclosure are capable of higher resolutions (e.g. 1.9 ⁇ 1.9 ⁇ 5 mm), as it is desirable to have a resolution high enough to develop a satisfactory image of a an average skull, which has an average thickness of 6.5-7.1 mm.
- Temporal resolutions different than 90 s are contemplated by the present disclosure, and it is therefore contemplated that the time per kz-encoding can be different than 1.13 s per kz encode in different embodiments of the present disclosure.
- the acceleration method can be any suitable acceleration method that can generate T1 mapping information within the desired clinical constraints.
- partial Fourier imaging can be applied. Partial Fourier imaging takes advantage of the conjugate symmetry of k-space applicable when the object is real or there are no phase errors, where
- and ⁇ x,y ⁇ ⁇ x, ⁇ y (same amplitude, opposite phase). In theory, only half of k-space needs to be acquired, but in practicality, phase errors do occur from B 0 -field inhomogeneities, concomitant gradients, and eddy currents. Thus, partial Fourier sampling can require acquisition of 60% or more of k-space. For UTE VIBE, 6/8 kz partial Fourier sampling was selected as a non-limiting example, and therefore the bottom 25% of k-space was not collected and scan time was reduced by approximately 25%.
- variable density spiral design samples the center of k-space at the Nyquist limit but under-samples the outer k-space regions reducing acquisition time. Because the center of k-space is fully-sampled and can contain most of the energy, under-sampling in outer k-space can lead to fewer artifacts than under-sampling uniformly. As spiral aliasing results in blurring instead of replicant overlap, under-sampling in the high spatial frequencies can lead to benign artifacts.
- step 104 includes determining a temperature change in the cortical bone based at least in part on a mathematical relationship established by T1 mapping thermometry produced according to the T1 mapping sequence.
- the temperature change in the cortical bone that occurs between the first point in time and the second point in time can be determined, where the temperature change is caused at least in part by a change in the FUS.
- T1-based thermometry Conventional MR thermometry does not work in the skull due to its ultra-short T2, so T1-based thermometry is used. Skull thermometry imaging should be relatively fast to capture temperature changes in clinically relevant timescales (e.g. the 90 s timeframe illustrated in FIG. 2 ). It is also beneficial for skull thermometry to be volumetric in order to detect heating anywhere in the skull, and have a short echo time ( ⁇ 100 us) to enable the imaging of bone. T1 is linear with temperature in cortical cow bone and can thus be calibrated to temperature. However, existing methods have not been demonstrated under clinical constraints and have a long acquisition time (8 minutes).
- T1-mapping thermometry can require twice as many acquisitions as T1-weighted thermometry, the T1 vs. temperature trend should be much more reliable and linear.
- the advantages of spiral MRI it is possible to accelerate T1-mapping to meet the clinical constraints (e.g. the constraints in FIG. 2 ).
- T 1 values from the UTE VIBE VFA method and the IR method were compared in FIGS. 10 A- 10 C .
- the IR values (shown in FIG. 10 C in solid black) were much closer to the expected T 1 based on NiCl 2 concentrations (mM).
- the mean difference in T 1 between VFA and IR was 6.39 ms (4.46% difference).
- the VFA values illustrated in FIG. 10 C (blue solid line) are less linear.
- IR may not be practical for UTE imaging; in IR, a 180° magnetization inversion must be achieved. Materials with short T2 such as cortical bone undergo relaxation during the inversion pulse thus making IR inefficient.
- the noisy VFA-T1 measurements can be corrected by performing a B 1 map to measure the actual flip angles rather than relying on the potentially erroneously prescribed flip angles.
- the UTE VIBE VFA method can be sensitive to T 1 with 5% error, enabling T1-mapping thermometry of cortical bone in step 104 .
- This T1-mapping thermometry has several useful clinical applications, including allowing the person or system administering FUS to a patient to either increase or decrease the intensity of the FUS and therefore determine the optimal level of FUS to apply to a patient to both treat the patient's condition and avoid unintentional damage to the surrounding tissue.
- the techniques described herein can be applied to portions of the brain that correspond to diseases including Parkinson's disease, essential tremor, neuropathic pain, depression, and obsessive-compulsive disorder, although the use of FUS to treat other conditions, while using T1 mapping thermometry, is contemplated by the present disclosure.
- thermometry was tested in simple conditions (cooling of bone in a water bath) and then in more challenging and clinically relevant conditions (heating of bone by focused ultrasound).
- T1-weighted thermometry The variable results of T1-weighted thermometry are a potential disadvantage compared to T1-mapping thermometry, which depends on less factors and assumptions but can take longer. T1-mapping with a better coil and increased resolution was also investigated. Analyzing both the T1 values and the T1-weighted signal at different flip angles, it was observed that the trend in T1-weighted signal is highly dependent on flip angle. Also, even with higher flip angles, T1-weighted signal is not fully linear with temperature. For the same ROIs, T1-mapping results showed a consistent linear trend (0.98+/ ⁇ 0.15 ms/° C.) whereas T1-weighted results showed mixed results.
- T1-mapping with the UTE VIBE was observed to be reliable, linear, and potentially able to be calibrated to indicate skull temperature.
- T1-mapping To accelerate T1-mapping, a 6/8 partial kz sampling was used and the sampling density of the spiral interleaves was changed using linear variable density with full sampling (1) at the center of k-space and 0.7 at the edge of k-space.
- the under-sampled T1 of bone cooled in a water bath still showed linear results, though the slope was higher than the fully-sampled T1 of other bones.
- Under-sampled T1-mapping was also done in ex-vivo human skull with results highly dependent on ROI due to the thinness of the skull and relatively coarse resolution.
- UTE VIBE T1 mapping thermometry is promising in its clinical applicability to skull monitoring, as preliminary results have shown linear measurements of T1 with temperature in contrast with the variable results of T1 weighted thermometry.
- FIGS. 13 A- 13 B An embodiment of the present disclosure was tested by heating and cooling bone samples in a water bath, and non-limiting examples are described herein.
- T1-mapping thermometry To test T1-mapping thermometry, several trials both with heating bone and cooling bone in a water bath were conducted.
- cooling experiments e.g. experiments 1 and 2 illustrated in FIGS. 13 A- 13 B
- cortical bovine bone was placed into a small plastic container filled with water heating to ⁇ 70° C. and equilibrated for 10 min.
- the long axis of the bone was aligned with the scanner and imaged transaxially with an L7 coil as it cooled with an improvement in SNR due to the proximity of the coil to the sample. ( FIGS. 11 A- 11 B ).
- Hysteresis of bone heating was tested by imaging the bone during heating using a water heater and a pump to see whether the change in T1 during heating was comparable to the change in T1 during cooling.
- a custom setup was used as shown in FIG. 12 A .
- the bone was placed into a small jar closed off from the outer jar.
- the circulated water was heated from room temperature up to 53° C. in ⁇ 4° C. increments.
- the bone and water in the small jar slowly heated in response to the surrounding water leading to gradual temperature changes (yellow trend slowly increases compared to the grey spikes of the circulating water in FIG. 12 B .
- a drill press was used to cut the bone into a smooth round shape which allowed it to fit into the jar.
- FIGS. 13 A- 13 B The T1 measured from the same ROI (same color) using two flip angles from the VFA method are also shown, illustrate the relationships between different T1 signals and temperature in bone for various experiments, wherein FIG. 13 A illustrates the relationship between changes in T1-weighted signal vs. temperature in bone, and FIG.
- T1 vs. temperature in Bone illustrates the relationship between a change in T1 vs. temperature in Bone.
- the T1-weighted signal is nonlinear, the corresponding T1 vs temperature values are linear, increasing with temperature (average slope of 0.98+/ ⁇ 0.15 ms/° C.), which is comparable to Han et al.'s result of 0.84 ms/° C. measured using a slower 3D radial UTE pulse sequence.
- Ex-vivo bovine femur bone was placed in a container of hot water and imaged as it cooled with a thermocouple measuring temperature in the bone. The signal was measured for three different flip angles (8°, 20°, 43.5°) at each temperature point. As T 1 increases with temperature, the T 1 weighted signal should decrease linearly in accordance to Eq. 7 for all flip angles. As shown in FIG. 14 A , the 8° FA data would show a smaller slope compared to the 43.5° FA. However, in the results illustrated in FIG. 14 B , a mix of trends was observed.
- a cooling experiment was conducted using an embodiment of the present disclosure for cow bone cooling in a water bath with an under-sampled UTE VIBE sequence.
- a linear variable density was chosen (1.1 to 0.7) with 6/8 partial kz, 105 interleaves, (1.625, 1.625, 5 mm) resolution lead to a 1.11 s/kz-encode time ( ⁇ 90 s for two flip angles).
- a linear T1 trend was observed with reasonable bone T1 values ( FIGS. 15 A- 15 B ).
- Previous sequences had a TA of 7.71 s/slice with higher resolution; an acceleration by ⁇ 7 times still allows for a measurement of linear T1 changes.
- T1-VFA based thermometry is feasible with spiral variable-density acceleration.
- T1 mapping accuracy of the UTE VIBE variable flip angle method was tested by using a NiCl 2 phantom.
- T 1 was initially measured using an inversion recovery (IR) 2D turbo spin echo sequence (TSE) to provide a ground truth comparison with VFA.
- IR inversion recovery
- TSE turbo spin echo sequence
- a cooling experiment was conducted for cow bone cooling in a water bath with an under-sampled UTE VIBE sequence.
- a linear variable density was chosen (1.1 to 0.7) with 6/8 partial kz, 105 interleaves, (1.625, 1.625, 5 mm) resolution leading to a 1.11 s/kz-encode time ( ⁇ 90 s for two flip angles).
- a linear T1 trend was observed with reasonable bone T1 values ( FIG. 15 B ).
- Previous sequences ( FIGS. 13 A- 13 B ) had a TA of 7.71 s/slice with higher resolution; an acceleration by ⁇ 7 times still allows for a measurement of linear T1 changes.
- T1-VFA based thermometry is feasible with spiral variable-density acceleration.
- FIGS. 18 A- 18 B Different ROIs within the skull showed different T1 vs. temperature trends ( FIGS. 18 A- 18 B ). This could be due to the porosity of the skull with pockets of water in the skull walls. Also, due to the large volume of the skull, the flip angle could vary across the skull and a flip angle correction map should be generated. The baseline T1 value was reasonable and in general either no trend or positive trends in T1 were observed. Repeating this experiment with a higher resolution as the skull is only a few pixels across and potentially with a fresher skull could improve results.
- MR bone thermometry should be able to detect localized heating caused by FUS.
- a small animal FUS transducer from was used.
- a 4-channel flex coil was used for imaging as the L7 coil (which can be better for this application) could not be used with the FUS setup; the L7 coil requires the use of a spine coil or another L7 coil, and this was not realized until after the experiment. Bone was gradually heated with 8 W for 20 min and imaged; however, images during sonication had strong artifacts. After reaching 53 C, bone was imaged while cooling as shown in FIG. 19 .
- T1 mapping thermometry One relevant clinical goal of T1 mapping thermometry according to some embodiments of the present disclosure is to detect localized heating.
- One embodiment of the present disclosure that was tested used UTE VIBE T1 thermometry. Bone was placed in Fomblin and heated with focused ultrasound with the results illustrated in FIGS. 20 A- 20 C . A flip angle miscalibration occurred in this experiment but a change in T1 was still detected (0.39 ms/° C.), though less than in previous experiments (0.98 ms/° C.).
- a NiCl 2 phantom was placed on top of an unfilled (no water) FUS transducer and imaged. Then the water tank was filled and the phantom was imaged again. The T1 decreased significantly ( FIGS. 22 A- 22 D ).
- the reference voltage was compared between the (no water tank) FUS setup (255V) and (water) FUS setup (201V); the difference in reference voltage indicated a different B1 calibration readjusted for the water tank which is in turn maladjusted for the bone. Manual RF calibration is thus needed to tune the B1 transmit for the phantom or bone.
- MRgFUS is an important medical technology enabling high-precision non-invasive brain surgery with ultrasound.
- medical applications include FDA approved treatment for Parkinson's disease and essential tremor and many other disorders in the research stage such as neuropathic pain, depression, and obsessive-compulsive disorder.
- One challenge to treatment efficacy is posed by skull heating. Temperature monitoring of the skull would increase treatment safety, enable further development of MRgFUS therapy to non-central brain targets, and potentially speed up treatment by decreasing waiting time between sonications for patients.
- T1-based thermometry Conventional MR thermometry does not work in the skull due to its ultra-short T2, so T1-based thermometry was used. Skull thermometry imaging is generally be fast to capture heating in 90 s, volumetric to detect heating anywhere in the skull, and have a short echo time ( ⁇ 100 us) to enable the imaging of bone. T1 is linear with temperature in cortical cow bone and can thus be calibrated to temperature. However, existing methods have not been demonstrated under clinical constraints and have a long acquisition times (e.g. 8 minutes). Embodiments of the present disclosure employ T1-weighted thermometry using a non-selective ultra-short-echo-time (UTE) 3D spiral sequence. First, T1-weighted thermometry was tested in simpler conditions (cooling of bone in a water bath) and then in more challenging clinically relevant conditions (heating of bone by focused ultrasound).
- UTE non-selective ultra-short-echo-time
- T1-mapping results showed a consistent linear trend (0.98+/ ⁇ 0.15 ms/° C.).
- T1-mapping with the UTE VIBE was observed to be reliable, linear, and potentially able to be calibrated to indicate skull temperature.
- T1-mapping can be accelerated as part of clinical application.
- 6/8 partial kz sampling was used and changed the sampling density of the spiral interleaves using linear variable density with full sampling (1) at the center of k-space and 0.7 at the edge of k-space.
- the under-sampled T1 of bone cooled in a water bath still showed linear results, though the slope was higher the fully-sampled T1 of other bones.
- thermometry is repeatable and reliable, and that it may be accelerated to potentially meet the clinical constraints (large FOV and short acquisition time).
- Manual RF calibration combined with a double angle B1 map to check the actual flip angle can also be performed.
- the use of localized FUS experiments for calibration is also contemplated, for example several trials of localized FUS experiments with L7 coils can be performed (with fat suppression and B1 mapping) on bovine bone. Then, the slope of those trials can serve as a calibration factor to convert T1 onto temperature for another “test” trial to determine method accuracy.
- the method described herein can be applied to ex-vivo skull experiments, porcine head experiments, and patients.
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Abstract
Description
τc is also inversely proportional to temperature, so T1 also approximately increases linearly with temperature within the clinical regime:
where Mxy is the measured signal; Mo is the thermal equilibrium magnetization; α is the flip angle; and TR is the repetition time. The e−T
where S(TE(kz)) is the signal intensity after a z-encoding of duration td.
S(k x(t),k y(t),k z(t))=e −t/T*
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